In some sensing and processing applications more than one quantity has to be measured by an electronic processing system. For some applications the absolute accuracy of the measure of each quantity is less important than the accuracy of one or more quantities relative to one or more other quantities.
For example in a control system where it is necessary to establish whether one quantity is bigger than another with a high degree of precision, so that the correct answer is given even if the larger input is for example only 0.1% bigger than the smaller, it is possible to use a two channel measurement system with absolute accuracy better than 0.05% for each channel in order to achieve the desired result. However, this absolute accuracy must be maintained under variations in the environment, such as time, temperature, pressure, and humidity. Such a high precision and stable system is expensive to produce because it requires expensive, close-tolerance, and stable components.
In an electrical system where it is desirable to measure or monitor the impedance or part thereof of a device, in the prior art it has been necessary to measure the current and voltage and to take the ratio in order to determine the impedance or part thereof in order to identify a change in impedance or part thereof. Such a device can for example comprise a proximity sensor. Such a sensor is disclosed in WO 01/65695, the content so which are hereby incorporated by reference. A sensor of this type provides a proximity measure whereby when a target is in the proximity of the device the impedance (or a component of impedance) of the device changes. WO 01/65695 discloses a method of proximity sensing in which the impedance of the proximity sensor is measured in order to detect a change of impedance. A known measurement system for use with proximity sensors such as that disclosed in WO 01/65695 is illustrated in FIG. 1. The proximity sensor 1 has an impedance Z which is required to be measured. The proximity sensor 1 is connected to earth and to a load resistance 2. A processor 3 is provided to digitally generate a drive wave form which is digital-to-analogue converted in a digital-to-analogue converter 4 to generate an analogue drive signal which is input through a low pass filter 5 and amplified by an amplifier 6 before being applied across the series connected load resistance 2 and proximity sensor 1. A voltage νS is measured as the voltage across the proximity sensor 1 by connecting a first analogue-to-digital converter 7 via a low pass filter 8. The output of the analogue-to-digital converter 7 is input to the processor 3. A second voltage νo is measured as the voltage across the series connected load resistance 2 and the proximity sensor 1. A second analogue-to-digital converter 9 receives the voltage signal νo via a second low pass filter 10 and inputs a digital representation of a voltage νo.
In the prior art proximity sensor such as disclosed in WO 01/65695 the sensor typically consists of a coil with or without a core of permeable material. The provision of the core allows directing of the magnetic flux of the coil and enhancement of the inductance. Directing of the flux allows the designer to have control over the direction of sensitivity of the centre assembly.
As can be seen in FIG. 1, the sensor 1 is driven by a drive signal having a voltage and a resulting current which is of fluctuating nature and which can be sinusoidal or transient to allow the impedance to be evaluated.
If a sinusoidal voltage at angular frequency ω is applied, the voltage and resulting current are expressed as:νo(t)=real (Vo(ω)ejαx)andi(t)=real (I(ω))ejαx)where Vo(ω) and I(ω) are the complex amplitudes. All of the voltages and currents in the circuit, not just these two, can be similarly expressed via complex amplitudes.
Impedance Z(ω) at the frequency ω is defined as:Z(ω)=Vs(ω)/I(ω).Also, impedance Z(ω) comprises:Z(ω)=R+jωL.orZ(ω)=R+1/(jωC).where R is the resistive component (the real component of impedance), L is the inductive component, and C is the capacitive component. It should be noted that the imaginary part of the impedance is frequency dependent and is usually termed the reactive component.
The portion of the current that is in phase with the voltage is often referred to as real or dissipative (or simply in phase) and is associated with the resistive behaviour of the coil and cables. The portion of the current that is in ‘phase quadrature’ or at 90 degrees to the voltage is often referred to as the imaginary, quadrature, or reactive component. It is associated with energy storage in the coil's electromagnetic fields, either due to its self-capacitance (electrostatic energy storage) or inductance (magnetic energy storage), or both.
The current is normally sensed by measuring the voltage produced by the current flowing through a known constant resistance in series with the coil and by using Ohm's law:I(ω))=ΔV(ω)/Rwhere ΔV(ω)) is the complex amplitude of the voltage across the load resistance 2.
It can thus be seen from FIG. 1 that in order to determine the impedance the νs measure provides the complex voltage applied to the sensor 1 and the current can be determined from the voltage difference between the voltage measures νo and νs and the value of the load resistance 2.
Thus within the processor 3, the simultaneous measurements of νo and νs are used to determine the complex voltage and current applied to the sensor 1 in order to determine an impedance value for the sensor 1.
The impedance and/or inductance of the sensor's coil are altered by the presence of a nearby metallic target. The target alters inductance in the following ways. If a target is made from a material with permeability greater than the normal medium surrounding the coil, then its presence enhances the magnetic field coupling through and around the coil, and so increases its inductance. It should be noted that the permeability of some materials is affected to some degree both by the excitation frequency, by temperature, and by other magnetic fields. If the target is a permanent magnet, then drawing it near to the core induces magnetic flux in the core. This forces the core nearer to its magnetic saturation which, with well-behaved magnetization or ‘BH’ curves, reduces core permeability. The reduction in permeability appears as a reduction in the reactive component of the impedance and hence reduction in inductance. If the target is made from a conductive material, the eddy currents induced in the target by the fluctuating magnetic field of the coil generate their own field, which substantially opposes that produced by the coil. The net field appears as a reduced field to the coil, which incurs a reduction in inductance as the target approaches the coil. In addition, the eddy currents in the target introduce energy loss that appears in the sensor's impedance as an increase in resistance.
Thus in the prior art in order to detect a change in inductance of the sensor 1, a very accurate load resistor 2 is placed in series with the sensor 1 to act as a current sensor. Two voltage measurements are taken across the load resistor 2 simultaneously using two measurement channels comprising low pass filters 8, 10 and analogue-to-digital converters 7, 9. Within the processor 3 a value for the impedance Z is calculated by taking a ratio of the processed measurements. Thus in order to determine the impedance to high accuracy, it is necessary for the measurement channels to be closely matched and to include high precision components. Further, the prior art system requires an expensive, close-tolerance, series sense resistor to provide for current sensing. In addition, computation implemented within the processor 3 requires a division for computation of impedance. Such a division is computationally expensive and requires protection from out of range results such as divide by 0.